As LSI integration densities have been becoming higher in recent years, miniaturization of internal circuit patterns is progressing. Along with the miniaturization, interconnect cross-sectional areas become smaller, and accordingly, interconnect resistances become higher. Also, the distances between adjacent interconnects become shorter, and accordingly, the interconnect capacitance between interconnects increases.
As a result, the interconnect delay time determined from the interconnect resistance and the interconnect capacitance becomes longer, and it becomes difficult to realize higher-speed LSIs. Also, as more and more LSIs have multi-core structures and three-dimensional integration of memory is progressing, large-capacity transmission is now imperative between cores or between cores and memory, and the speed of transmission with electrical signals is the bottleneck in further improvement of LSIs.
An optical interconnect technique by which electrical signals are replaced with optical signals is now drawing attention as a technique for solving the interconnect delay problem accompanying higher-density LSIs. The optical interconnect technique is a method of transmitting signals by using an optical waveguide, instead of metal interconnects. The optical interconnect technique does not cause increases in interconnect resistance and capacitance between interconnects due to the above described miniaturization, and higher operation speeds can be expected with such a technique.
A semiconductor laser is used as a light emitting element that is used as a light source according to the optical interconnect technique. The semiconductor lasers used in conventional optical communications each have a size of several μm and a length of a hundred μm, which are much larger compared with LSI transistors and interconnect pitch. Therefore, the size of each semiconductor laser is a major hindrance to replacement of electrical interconnects with optical interconnects. In view of this, attention is now being drawn to microring (or microdisk) lasers each using a resonator having a microring (or a microdisk) as a small-sized light source.
Also, to realize optical interconnects on an LSI chip, an optical transmission/reception system needs to be formed by integrating a light receiving element (a light receiving unit), a drive circuit, and amplifier circuit, as well as a light emitting element (an emitting unit) as a light source and an optical waveguide (a transmitting unit), on the same chip in a compact fashion.
In a case where a microring laser as a light emitting element and a light receiving element are integrated, it is desirable to form the light receiving element and the light emitting element with the same microring structures, so as to lower costs. By doing so, the light emitting element and the light receiving element can be simultaneously manufactured through the same process.
In a light emitting element and a light receiving element having a microring structure, a positive electrode and a negative electrode are provided above and below an active layer so that light emission and light reception can be performed by applying a bias voltage to the element. Examples of electrode structures include a structure having the lower electrode (normally the negative electrode) provided inside the microring (an internal electrode structure) and a structure having the lower electrode outside the microring (an external electrode structure).
Between those two, an internal electrode structure is effective with a relatively large ring (several tens of μm or larger). On the other hand, an external electrode structure is effective with a ring or a disk in which it is difficult or impossible to form an electrode, such as a ring of 10 um or smaller in diameter or a disk.
Particularly, miniaturization of microring structures is a critical issue not only in achieving higher integration but also in reducing power consumptions and increasing speeds of elements. Miniaturization of microring structures is also critical in wavelength division multiplexing (WDM) for achieving larger capacities.
For example, in a case where wavelength multiplexing is performed with a microring laser, oscillation wavelength can be easily varied by gradually changing the ring diameter. However, the variable wavelength range is normally one to two times larger than the free spectral range (FSR), so as to avoid crosstalk between wavelength channels. Therefore, the variable wavelength range becomes narrower as the ring becomes larger. The narrowing of the variable wavelength range leads to an increase in crosstalk, an increase in size of the multiplexing/demultiplexing device (the wavelength filter), and a reduction in the bandwidth.
As described above, an external electrode structure can be considered suitable for reductions in the size and power consumption of a light emitting element or a light receiving element having a microring structure, and for an increase in the capacity through WDM.